Seismic wave monitoring of CO2 migration in water-saturated porous sandstone

Abstract

We have carried out laboratory measurements of P-wave velocity and deformation strain during CO2 injection into a porous sandstone sample, in dry and water-saturated conditions. The rock sample was cylindrical, with the axis normal to the bedding plane, and fluid injection was performed from one end. Using a piezoelectric transducer array system, we mapped fluid movement during injection of distilled water into dry sandstone, and of gaseous, liquid, and supercritical CO2 into a water-saturated sample. The velocity changes caused by water injection ranged from 5.61 to 7.52%. The velocity changes caused by CO2 injection are typically about -6%, and about -10% for injection of supercritical CO2. Such changes in velocity show that the seismic method may be useful in mapping CO2 movement in the subsurface. Strain normal to the bedding plane was greater than strain parallel to the bedding plane during CO2 injection; injection of supercritical CO2 showed a particularly strong effect. Strain changes suggest the possibility of monitoring rock mass deformation by using borehole tiltmeters at geological sequestration sites. We also found differences associated with CO2 phases in velocity and strain changes during injection.